The thread rolling process has been widely accepted as a means of forming external threads accurately, quickly and inexpensively. Production rates have reached six hundred parts per minute. The product so manufactured has superior physical qualities to that produced by machining. Significant material savings are realized since a blank is utilized having an outside diameter which is less than the outside diameter of the threaded fastener to be formed. This invention relates primarily to flat dies for such thread rolling processes and particularly to dies for rolling those threaded fasteners which have both a straight section and a point section. Other applications may include planetary dies for the rolling of such dies.
Such dies have been relatively expensive to manufacture because of the necessity of making multiple passes either with a grinding wheel or a milling cutter to individually produce each groove. The individual machining or grinding has been necessary because the face surface of the die which contacted the workpiece comprises two generally planar surfaces; the first generally planar surface disposed on the face of the die in substantially parallel relationship to the axis of the straight section of the workpiece and the second generally planar surface also disposed on the face of the die which contacts the workpiece. As viewed from one end of the die, the angular relationship between the two generally planar surfaces is equal to the angle between the straight section and point section of the finished screw.
The intersection of the first and second generally planar surfaces defined a line parallel to the direction of relative movement between the pair of dies utilized to form a single screw. Because the ridges disposed on the first and second generally planar surfaces of the face are obliquely disposed with respect to the intersection of the first and second generally planar surfaces, each ridge in the first generally planar surface was disposed in the same plane as the intersection of the first and second generally planar surfaces, which is sometimes referred to as the heel line, and each ridge is disposed at an angle to the direction of movement substantially equal to the helix angle of the thread to be rolled. Each ridge on the second generally planar surface is an extension of a ridge on the first generally planar surface. It is not feasible to use a multipoint cutter or grinding wheel to simultaneously generate all of the ridges in this configuration because the multipoint tool, as it progresses obliquely with respect to the heel line, will generate grooves of unequal depth if any attempt is made to obliquely pass over the "heel line."
The single point tools which have been typically utilized are controlled by means of cams or other mechanical apparatus to follow a predetermined rise and fall path which imparts a contoured thread forming surface into the die working face. The number of milling cuts or grinding cuts that must be made in single point generation of the die working face is determined by the length and pitch of the thread which is to be rolled by the die being made.
It is apparent that the mechanism for imparting the described rise and fall path, as well as the time involved in repetitively cutting each individual groove while describing the referred to rise and fall path, results in a relatively expensive manufacturing process.